/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.beans.factory.support;

import java.io.IOException;
import java.io.NotSerializableException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;
import java.lang.annotation.Annotation;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.lang.reflect.Method;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.IdentityHashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import javax.inject.Provider;

import org.springframework.beans.BeanUtils;
import org.springframework.beans.BeansException;
import org.springframework.beans.TypeConverter;
import org.springframework.beans.factory.BeanCreationException;
import org.springframework.beans.factory.BeanCurrentlyInCreationException;
import org.springframework.beans.factory.BeanDefinitionStoreException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.BeanNotOfRequiredTypeException;
import org.springframework.beans.factory.CannotLoadBeanClassException;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.beans.factory.InjectionPoint;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
import org.springframework.beans.factory.ObjectFactory;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.beans.factory.SmartFactoryBean;
import org.springframework.beans.factory.SmartInitializingSingleton;
import org.springframework.beans.factory.config.AutowireCapableBeanFactory;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.BeanDefinitionHolder;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.DependencyDescriptor;
import org.springframework.beans.factory.config.NamedBeanHolder;
import org.springframework.core.OrderComparator;
import org.springframework.core.ResolvableType;
import org.springframework.core.annotation.AnnotationUtils;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.CompositeIterator;
import org.springframework.util.ObjectUtils;
import org.springframework.util.StringUtils;

/**
 * Default implementation of the
 * {@link org.springframework.beans.factory.ListableBeanFactory} and
 * {@link BeanDefinitionRegistry} interfaces: a full-fledged bean factory
 * based on bean definition objects.
 *
 * <p>Typical usage is registering all bean definitions first (possibly read
 * from a bean definition file), before accessing beans. Bean definition lookup
 * is therefore an inexpensive operation in a local bean definition table,
 * operating on pre-built bean definition metadata objects.
 *
 * <p>Can be used as a standalone bean factory, or as a superclass for custom
 * bean factories. Note that readers for specific bean definition formats are
 * typically implemented separately rather than as bean factory subclasses:
 * see for example {@link PropertiesBeanDefinitionReader} and
 * {@link org.springframework.beans.factory.xml.XmlBeanDefinitionReader}.
 *
 * <p>For an alternative implementation of the
 * {@link org.springframework.beans.factory.ListableBeanFactory} interface,
 * have a look at {@link StaticListableBeanFactory}, which manages existing
 * bean instances rather than creating new ones based on bean definitions.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @author Costin Leau
 * @author Chris Beams
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @see StaticListableBeanFactory
 * @see PropertiesBeanDefinitionReader
 * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
 * @since 16 April 2001
 */
@SuppressWarnings("serial")
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
        implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {

    /**
     * Map from serialized id to factory instance
     */
    private static final Map<String, Reference<DefaultListableBeanFactory>> serializableFactories =
            new ConcurrentHashMap<>(8);
    @Nullable
    private static Class<?> javaxInjectProviderClass;

    static {
        try {
            javaxInjectProviderClass =
                    ClassUtils.forName("javax.inject.Provider", DefaultListableBeanFactory.class.getClassLoader());
        } catch (ClassNotFoundException ex) {
            // JSR-330 API not available - Provider interface simply not supported then.
            javaxInjectProviderClass = null;
        }
    }

    /**
     * Map from dependency type to corresponding autowired value
     */
    private final Map<Class<?>, Object> resolvableDependencies = new ConcurrentHashMap<>(16);
    /**
     * Map of bean definition objects, keyed by bean name
     */
    private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
    /**
     * Map of singleton and non-singleton bean names, keyed by dependency type
     */
    private final Map<Class<?>, String[]> allBeanNamesByType = new ConcurrentHashMap<>(64);
    /**
     * Map of singleton-only bean names, keyed by dependency type
     */
    private final Map<Class<?>, String[]> singletonBeanNamesByType = new ConcurrentHashMap<>(64);
    /**
     * Optional id for this factory, for serialization purposes
     */
    @Nullable
    private String serializationId;
    /**
     * Whether to allow re-registration of a different definition with the same name
     */
    private boolean allowBeanDefinitionOverriding = true;
    /**
     * Whether to allow eager class loading even for lazy-init beans
     */
    private boolean allowEagerClassLoading = true;
    /**
     * Optional OrderComparator for dependency Lists and arrays
     */
    @Nullable
    private Comparator<Object> dependencyComparator;
    /**
     * Resolver to use for checking if a bean definition is an autowire candidate
     */
    private AutowireCandidateResolver autowireCandidateResolver = new SimpleAutowireCandidateResolver();
    /**
     * List of bean definition names, in registration order
     */
    private volatile List<String> beanDefinitionNames = new ArrayList<>(256);

    /**
     * List of names of manually registered singletons, in registration order
     */
    private volatile Set<String> manualSingletonNames = new LinkedHashSet<>(16);

    /**
     * Cached array of bean definition names in case of frozen configuration
     */
    @Nullable
    private volatile String[] frozenBeanDefinitionNames;

    /**
     * Whether bean definition metadata may be cached for all beans
     */
    private volatile boolean configurationFrozen = false;


    /**
     * Create a new DefaultListableBeanFactory.
     */
    public DefaultListableBeanFactory() {
        super();
    }

    /**
     * Create a new DefaultListableBeanFactory with the given parent.
     *
     * @param parentBeanFactory the parent BeanFactory
     */
    public DefaultListableBeanFactory(@Nullable BeanFactory parentBeanFactory) {
        super(parentBeanFactory);
    }

    /**
     * Return an id for serialization purposes, if specified, allowing this BeanFactory
     * to be deserialized from this id back into the BeanFactory object, if needed.
     *
     * @since 4.1.2
     */
    @Nullable
    public String getSerializationId() {
        return this.serializationId;
    }

    /**
     * Specify an id for serialization purposes, allowing this BeanFactory to be
     * deserialized from this id back into the BeanFactory object, if needed.
     */
    public void setSerializationId(@Nullable String serializationId) {
        if (serializationId != null) {
            serializableFactories.put(serializationId, new WeakReference<>(this));
        } else if (this.serializationId != null) {
            serializableFactories.remove(this.serializationId);
        }
        this.serializationId = serializationId;
    }

    /**
     * Return whether it should be allowed to override bean definitions by registering
     * a different definition with the same name, automatically replacing the former.
     *
     * @since 4.1.2
     */
    public boolean isAllowBeanDefinitionOverriding() {
        return this.allowBeanDefinitionOverriding;
    }

    /**
     * Set whether it should be allowed to override bean definitions by registering
     * a different definition with the same name, automatically replacing the former.
     * If not, an exception will be thrown. This also applies to overriding aliases.
     * <p>Default is "true".
     *
     * @see #registerBeanDefinition
     */
    public void setAllowBeanDefinitionOverriding(boolean allowBeanDefinitionOverriding) {
        this.allowBeanDefinitionOverriding = allowBeanDefinitionOverriding;
    }

    /**
     * Return whether the factory is allowed to eagerly load bean classes
     * even for bean definitions that are marked as "lazy-init".
     *
     * @since 4.1.2
     */
    public boolean isAllowEagerClassLoading() {
        return this.allowEagerClassLoading;
    }

    /**
     * Set whether the factory is allowed to eagerly load bean classes
     * even for bean definitions that are marked as "lazy-init".
     * <p>Default is "true". Turn this flag off to suppress class loading
     * for lazy-init beans unless such a bean is explicitly requested.
     * In particular, by-type lookups will then simply ignore bean definitions
     * without resolved class name, instead of loading the bean classes on
     * demand just to perform a type check.
     *
     * @see AbstractBeanDefinition#setLazyInit
     */
    public void setAllowEagerClassLoading(boolean allowEagerClassLoading) {
        this.allowEagerClassLoading = allowEagerClassLoading;
    }

    /**
     * Return the dependency comparator for this BeanFactory (may be {@code null}.
     *
     * @since 4.0
     */
    @Nullable
    public Comparator<Object> getDependencyComparator() {
        return this.dependencyComparator;
    }

    /**
     * Set a {@link java.util.Comparator} for dependency Lists and arrays.
     *
     * @see org.springframework.core.OrderComparator
     * @see org.springframework.core.annotation.AnnotationAwareOrderComparator
     * @since 4.0
     */
    public void setDependencyComparator(@Nullable Comparator<Object> dependencyComparator) {
        this.dependencyComparator = dependencyComparator;
    }

    /**
     * Return the autowire candidate resolver for this BeanFactory (never {@code null}).
     */
    public AutowireCandidateResolver getAutowireCandidateResolver() {
        return this.autowireCandidateResolver;
    }

    /**
     * Set a custom autowire candidate resolver for this BeanFactory to use
     * when deciding whether a bean definition should be considered as a
     * candidate for autowiring.
     */
    public void setAutowireCandidateResolver(final AutowireCandidateResolver autowireCandidateResolver) {
        Assert.notNull(autowireCandidateResolver, "AutowireCandidateResolver must not be null");
        if (autowireCandidateResolver instanceof BeanFactoryAware) {
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                    ((BeanFactoryAware) autowireCandidateResolver).setBeanFactory(DefaultListableBeanFactory.this);
                    return null;
                }, getAccessControlContext());
            } else {
                ((BeanFactoryAware) autowireCandidateResolver).setBeanFactory(this);
            }
        }
        this.autowireCandidateResolver = autowireCandidateResolver;
    }

    @Override
    public void copyConfigurationFrom(ConfigurableBeanFactory otherFactory) {
        super.copyConfigurationFrom(otherFactory);
        if (otherFactory instanceof DefaultListableBeanFactory) {
            DefaultListableBeanFactory otherListableFactory = (DefaultListableBeanFactory) otherFactory;
            this.allowBeanDefinitionOverriding = otherListableFactory.allowBeanDefinitionOverriding;
            this.allowEagerClassLoading = otherListableFactory.allowEagerClassLoading;
            this.dependencyComparator = otherListableFactory.dependencyComparator;
            // A clone of the AutowireCandidateResolver since it is potentially BeanFactoryAware...
            setAutowireCandidateResolver(BeanUtils.instantiateClass(getAutowireCandidateResolver().getClass()));
            // Make resolvable dependencies (e.g. ResourceLoader) available here as well...
            this.resolvableDependencies.putAll(otherListableFactory.resolvableDependencies);
        }
    }


    //---------------------------------------------------------------------
    // Implementation of remaining BeanFactory methods
    //---------------------------------------------------------------------

    @Override
    public <T> T getBean(Class<T> requiredType) throws BeansException {
        return getBean(requiredType, (Object[]) null);
    }

    @Override
    public <T> T getBean(Class<T> requiredType, @Nullable Object... args) throws BeansException {
        NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args);
        if (namedBean != null) {
            return namedBean.getBeanInstance();
        }
        BeanFactory parent = getParentBeanFactory();
        if (parent != null) {
            return (args != null ? parent.getBean(requiredType, args) : parent.getBean(requiredType));
        }
        throw new NoSuchBeanDefinitionException(requiredType);
    }


    //---------------------------------------------------------------------
    // Implementation of ListableBeanFactory interface
    //---------------------------------------------------------------------

    @Override
    public boolean containsBeanDefinition(String beanName) {
        Assert.notNull(beanName, "Bean name must not be null");
        return this.beanDefinitionMap.containsKey(beanName);
    }

    @Override
    public int getBeanDefinitionCount() {
        return this.beanDefinitionMap.size();
    }

    @Override
    public String[] getBeanDefinitionNames() {
        String[] frozenNames = this.frozenBeanDefinitionNames;
        if (frozenNames != null) {
            return frozenNames.clone();
        } else {
            return StringUtils.toStringArray(this.beanDefinitionNames);
        }
    }

    @Override
    public String[] getBeanNamesForType(ResolvableType type) {
        return doGetBeanNamesForType(type, true, true);
    }

    @Override
    public String[] getBeanNamesForType(@Nullable Class<?> type) {
        return getBeanNamesForType(type, true, true);
    }

    @Override
    public String[] getBeanNamesForType(@Nullable Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
        if (!isConfigurationFrozen() || type == null || !allowEagerInit) {
            return doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, allowEagerInit);
        }
        Map<Class<?>, String[]> cache =
                (includeNonSingletons ? this.allBeanNamesByType : this.singletonBeanNamesByType);
        String[] resolvedBeanNames = cache.get(type);
        if (resolvedBeanNames != null) {
            return resolvedBeanNames;
        }
        resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);
        if (ClassUtils.isCacheSafe(type, getBeanClassLoader())) {
            cache.put(type, resolvedBeanNames);
        }
        return resolvedBeanNames;
    }

    private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit) {
        List<String> result = new ArrayList<>();

        // Check all bean definitions.
        for (String beanName : this.beanDefinitionNames) {
            // Only consider bean as eligible if the bean name
            // is not defined as alias for some other bean.
            if (!isAlias(beanName)) {
                try {
                    RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                    // Only check bean definition if it is complete.
                    if (!mbd.isAbstract() && (allowEagerInit ||
                            ((mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading())) &&
                                    !requiresEagerInitForType(mbd.getFactoryBeanName()))) {
                        // In case of FactoryBean, match object created by FactoryBean.
                        boolean isFactoryBean = isFactoryBean(beanName, mbd);
                        BeanDefinitionHolder dbd = mbd.getDecoratedDefinition();
                        boolean matchFound =
                                (allowEagerInit || !isFactoryBean ||
                                        (dbd != null && !mbd.isLazyInit()) || containsSingleton(beanName)) &&
                                        (includeNonSingletons ||
                                                (dbd != null ? mbd.isSingleton() : isSingleton(beanName))) &&
                                        isTypeMatch(beanName, type);
                        if (!matchFound && isFactoryBean) {
                            // In case of FactoryBean, try to match FactoryBean instance itself next.
                            beanName = FACTORY_BEAN_PREFIX + beanName;
                            matchFound = (includeNonSingletons || mbd.isSingleton()) && isTypeMatch(beanName, type);
                        }
                        if (matchFound) {
                            result.add(beanName);
                        }
                    }
                } catch (CannotLoadBeanClassException ex) {
                    if (allowEagerInit) {
                        throw ex;
                    }
                    // Probably contains a placeholder: let's ignore it for type matching purposes.
                    if (this.logger.isDebugEnabled()) {
                        this.logger.debug("Ignoring bean class loading failure for bean '" + beanName + "'", ex);
                    }
                    onSuppressedException(ex);
                } catch (BeanDefinitionStoreException ex) {
                    if (allowEagerInit) {
                        throw ex;
                    }
                    // Probably contains a placeholder: let's ignore it for type matching purposes.
                    if (this.logger.isDebugEnabled()) {
                        this.logger.debug("Ignoring unresolvable metadata in bean definition '" + beanName + "'", ex);
                    }
                    onSuppressedException(ex);
                }
            }
        }

        // Check manually registered singletons too.
        for (String beanName : this.manualSingletonNames) {
            try {
                // In case of FactoryBean, match object created by FactoryBean.
                if (isFactoryBean(beanName)) {
                    if ((includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type)) {
                        result.add(beanName);
                        // Match found for this bean: do not match FactoryBean itself anymore.
                        continue;
                    }
                    // In case of FactoryBean, try to match FactoryBean itself next.
                    beanName = FACTORY_BEAN_PREFIX + beanName;
                }
                // Match raw bean instance (might be raw FactoryBean).
                if (isTypeMatch(beanName, type)) {
                    result.add(beanName);
                }
            } catch (NoSuchBeanDefinitionException ex) {
                // Shouldn't happen - probably a result of circular reference resolution...
                if (logger.isDebugEnabled()) {
                    logger.debug("Failed to check manually registered singleton with name '" + beanName + "'", ex);
                }
            }
        }

        return StringUtils.toStringArray(result);
    }

    /**
     * Check whether the specified bean would need to be eagerly initialized
     * in order to determine its type.
     *
     * @param factoryBeanName a factory-bean reference that the bean definition
     * defines a factory method for
     * @return whether eager initialization is necessary
     */
    private boolean requiresEagerInitForType(@Nullable String factoryBeanName) {
        return (factoryBeanName != null && isFactoryBean(factoryBeanName) && !containsSingleton(factoryBeanName));
    }

    @Override
    public <T> Map<String, T> getBeansOfType(@Nullable Class<T> type) throws BeansException {
        return getBeansOfType(type, true, true);
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T> Map<String, T> getBeansOfType(@Nullable Class<T> type, boolean includeNonSingletons,
            boolean allowEagerInit)
            throws BeansException {

        String[] beanNames = getBeanNamesForType(type, includeNonSingletons, allowEagerInit);
        Map<String, T> result = new LinkedHashMap<>(beanNames.length);
        for (String beanName : beanNames) {
            try {
                Object beanInstance = getBean(beanName);
                result.put(beanName, (beanInstance instanceof NullBean ? null : (T) beanInstance));
            } catch (BeanCreationException ex) {
                Throwable rootCause = ex.getMostSpecificCause();
                if (rootCause instanceof BeanCurrentlyInCreationException) {
                    BeanCreationException bce = (BeanCreationException) rootCause;
                    String exBeanName = bce.getBeanName();
                    if (exBeanName != null && isCurrentlyInCreation(exBeanName)) {
                        if (this.logger.isDebugEnabled()) {
                            this.logger.debug("Ignoring match to currently created bean '" + exBeanName + "': " +
                                    ex.getMessage());
                        }
                        onSuppressedException(ex);
                        // Ignore: indicates a circular reference when autowiring constructors.
                        // We want to find matches other than the currently created bean itself.
                        continue;
                    }
                }
                throw ex;
            }
        }
        return result;
    }

    @Override
    public String[] getBeanNamesForAnnotation(Class<? extends Annotation> annotationType) {
        List<String> results = new ArrayList<>();
        for (String beanName : this.beanDefinitionNames) {
            BeanDefinition beanDefinition = getBeanDefinition(beanName);
            if (!beanDefinition.isAbstract() && findAnnotationOnBean(beanName, annotationType) != null) {
                results.add(beanName);
            }
        }
        for (String beanName : this.manualSingletonNames) {
            if (!results.contains(beanName) && findAnnotationOnBean(beanName, annotationType) != null) {
                results.add(beanName);
            }
        }
        return StringUtils.toStringArray(results);
    }

    @Override
    public Map<String, Object> getBeansWithAnnotation(Class<? extends Annotation> annotationType) {
        String[] beanNames = getBeanNamesForAnnotation(annotationType);
        Map<String, Object> results = new LinkedHashMap<>(beanNames.length);
        for (String beanName : beanNames) {
            results.put(beanName, getBean(beanName));
        }
        return results;
    }

    /**
     * Find a {@link Annotation} of {@code annotationType} on the specified
     * bean, traversing its interfaces and super classes if no annotation can be
     * found on the given class itself, as well as checking its raw bean class
     * if not found on the exposed bean reference (e.g. in case of a proxy).
     */
    @Override
    @Nullable
    public <A extends Annotation> A findAnnotationOnBean(String beanName, Class<A> annotationType)
            throws NoSuchBeanDefinitionException {

        A ann = null;
        Class<?> beanType = getType(beanName);
        if (beanType != null) {
            ann = AnnotationUtils.findAnnotation(beanType, annotationType);
        }
        if (ann == null && containsBeanDefinition(beanName)) {
            BeanDefinition bd = getMergedBeanDefinition(beanName);
            if (bd instanceof AbstractBeanDefinition) {
                AbstractBeanDefinition abd = (AbstractBeanDefinition) bd;
                if (abd.hasBeanClass()) {
                    ann = AnnotationUtils.findAnnotation(abd.getBeanClass(), annotationType);
                }
            }
        }
        return ann;
    }


    //---------------------------------------------------------------------
    // Implementation of ConfigurableListableBeanFactory interface
    //---------------------------------------------------------------------

    @Override
    public void registerResolvableDependency(Class<?> dependencyType, @Nullable Object autowiredValue) {
        Assert.notNull(dependencyType, "Dependency type must not be null");
        if (autowiredValue != null) {
            if (!(autowiredValue instanceof ObjectFactory || dependencyType.isInstance(autowiredValue))) {
                throw new IllegalArgumentException("Value [" + autowiredValue +
                        "] does not implement specified dependency type [" + dependencyType.getName() + "]");
            }
            this.resolvableDependencies.put(dependencyType, autowiredValue);
        }
    }

    @Override
    public boolean isAutowireCandidate(String beanName, DependencyDescriptor descriptor)
            throws NoSuchBeanDefinitionException {

        return isAutowireCandidate(beanName, descriptor, getAutowireCandidateResolver());
    }

    /**
     * Determine whether the specified bean definition qualifies as an autowire candidate,
     * to be injected into other beans which declare a dependency of matching type.
     *
     * @param beanName the name of the bean definition to check
     * @param descriptor the descriptor of the dependency to resolve
     * @param resolver the AutowireCandidateResolver to use for the actual resolution algorithm
     * @return whether the bean should be considered as autowire candidate
     */
    protected boolean isAutowireCandidate(String beanName, DependencyDescriptor descriptor,
            AutowireCandidateResolver resolver)
            throws NoSuchBeanDefinitionException {

        String beanDefinitionName = BeanFactoryUtils.transformedBeanName(beanName);
        if (containsBeanDefinition(beanDefinitionName)) {
            return isAutowireCandidate(beanName, getMergedLocalBeanDefinition(beanDefinitionName), descriptor,
                    resolver);
        } else if (containsSingleton(beanName)) {
            return isAutowireCandidate(beanName, new RootBeanDefinition(getType(beanName)), descriptor, resolver);
        }

        BeanFactory parent = getParentBeanFactory();
        if (parent instanceof DefaultListableBeanFactory) {
            // No bean definition found in this factory -> delegate to parent.
            return ((DefaultListableBeanFactory) parent).isAutowireCandidate(beanName, descriptor, resolver);
        } else if (parent instanceof ConfigurableListableBeanFactory) {
            // If no DefaultListableBeanFactory, can't pass the resolver along.
            return ((ConfigurableListableBeanFactory) parent).isAutowireCandidate(beanName, descriptor);
        } else {
            return true;
        }
    }

    /**
     * Determine whether the specified bean definition qualifies as an autowire candidate,
     * to be injected into other beans which declare a dependency of matching type.
     *
     * @param beanName the name of the bean definition to check
     * @param mbd the merged bean definition to check
     * @param descriptor the descriptor of the dependency to resolve
     * @param resolver the AutowireCandidateResolver to use for the actual resolution algorithm
     * @return whether the bean should be considered as autowire candidate
     */
    protected boolean isAutowireCandidate(String beanName, RootBeanDefinition mbd,
            DependencyDescriptor descriptor, AutowireCandidateResolver resolver) {

        String beanDefinitionName = BeanFactoryUtils.transformedBeanName(beanName);
        resolveBeanClass(mbd, beanDefinitionName);
        if (mbd.isFactoryMethodUnique) {
            boolean resolve;
            synchronized (mbd.constructorArgumentLock) {
                resolve = (mbd.resolvedConstructorOrFactoryMethod == null);
            }
            if (resolve) {
                new ConstructorResolver(this).resolveFactoryMethodIfPossible(mbd);
            }
        }
        return resolver.isAutowireCandidate(
                new BeanDefinitionHolder(mbd, beanName, getAliases(beanDefinitionName)), descriptor);
    }

    @Override
    public BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException {
        BeanDefinition bd = this.beanDefinitionMap.get(beanName);
        if (bd == null) {
            if (this.logger.isTraceEnabled()) {
                this.logger.trace("No bean named '" + beanName + "' found in " + this);
            }
            throw new NoSuchBeanDefinitionException(beanName);
        }
        return bd;
    }

    @Override
    public Iterator<String> getBeanNamesIterator() {
        CompositeIterator<String> iterator = new CompositeIterator<>();
        iterator.add(this.beanDefinitionNames.iterator());
        iterator.add(this.manualSingletonNames.iterator());
        return iterator;
    }

    @Override
    public void clearMetadataCache() {
        super.clearMetadataCache();
        clearByTypeCache();
    }

    @Override
    public void freezeConfiguration() {
        this.configurationFrozen = true;
        this.frozenBeanDefinitionNames = StringUtils.toStringArray(this.beanDefinitionNames);
    }

    @Override
    public boolean isConfigurationFrozen() {
        return this.configurationFrozen;
    }

    /**
     * Considers all beans as eligible for metadata caching
     * if the factory's configuration has been marked as frozen.
     *
     * @see #freezeConfiguration()
     */
    @Override
    protected boolean isBeanEligibleForMetadataCaching(String beanName) {
        return (this.configurationFrozen || super.isBeanEligibleForMetadataCaching(beanName));
    }

    @Override
    public void preInstantiateSingletons() throws BeansException {
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Pre-instantiating singletons in " + this);
        }

        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

        // Trigger initialization of all non-lazy singleton beans...
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                if (isFactoryBean(beanName)) {
                    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                    if (bean instanceof FactoryBean) {
                        final FactoryBean<?> factory = (FactoryBean<?>) bean;
                        boolean isEagerInit;
                        if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                            isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                            ((SmartFactoryBean<?>) factory)::isEagerInit,
                                    getAccessControlContext());
                        } else {
                            isEagerInit = (factory instanceof SmartFactoryBean &&
                                    ((SmartFactoryBean<?>) factory).isEagerInit());
                        }
                        if (isEagerInit) {
                            getBean(beanName);
                        }
                    }
                } else {
                    getBean(beanName);
                }
            }
        }

        // Trigger post-initialization callback for all applicable beans...
        for (String beanName : beanNames) {
            Object singletonInstance = getSingleton(beanName);
            if (singletonInstance instanceof SmartInitializingSingleton) {
                final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
                if (System.getSecurityManager() != null) {
                    AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                        smartSingleton.afterSingletonsInstantiated();
                        return null;
                    }, getAccessControlContext());
                } else {
                    smartSingleton.afterSingletonsInstantiated();
                }
            }
        }
    }


    //---------------------------------------------------------------------
    // Implementation of BeanDefinitionRegistry interface
    //---------------------------------------------------------------------

    @Override
    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
            throws BeanDefinitionStoreException {

        Assert.hasText(beanName, "Bean name must not be empty");
        Assert.notNull(beanDefinition, "BeanDefinition must not be null");

        if (beanDefinition instanceof AbstractBeanDefinition) {
            try {
                ((AbstractBeanDefinition) beanDefinition).validate();
            } catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Validation of bean definition failed", ex);
            }
        }

        BeanDefinition oldBeanDefinition;

        oldBeanDefinition = this.beanDefinitionMap.get(beanName);
        if (oldBeanDefinition != null) {
            if (!isAllowBeanDefinitionOverriding()) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                                "': There is already [" + oldBeanDefinition + "] bound.");
            } else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
                // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                            "' with a framework-generated bean definition: replacing [" +
                            oldBeanDefinition + "] with [" + beanDefinition + "]");
                }
            } else if (!beanDefinition.equals(oldBeanDefinition)) {
                if (this.logger.isInfoEnabled()) {
                    this.logger.info("Overriding bean definition for bean '" + beanName +
                            "' with a different definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            } else {
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Overriding bean definition for bean '" + beanName +
                            "' with an equivalent definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            this.beanDefinitionMap.put(beanName, beanDefinition);
        } else {
            if (hasBeanCreationStarted()) {
                // Cannot modify startup-time collection elements anymore (for stable iteration)
                synchronized (this.beanDefinitionMap) {
                    this.beanDefinitionMap.put(beanName, beanDefinition);
                    List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
                    updatedDefinitions.addAll(this.beanDefinitionNames);
                    updatedDefinitions.add(beanName);
                    this.beanDefinitionNames = updatedDefinitions;
                    if (this.manualSingletonNames.contains(beanName)) {
                        Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
                        updatedSingletons.remove(beanName);
                        this.manualSingletonNames = updatedSingletons;
                    }
                }
            } else {
                // Still in startup registration phase
                this.beanDefinitionMap.put(beanName, beanDefinition);
                this.beanDefinitionNames.add(beanName);
                this.manualSingletonNames.remove(beanName);
            }
            this.frozenBeanDefinitionNames = null;
        }

        if (oldBeanDefinition != null || containsSingleton(beanName)) {
            resetBeanDefinition(beanName);
        }
    }

    @Override
    public void removeBeanDefinition(String beanName) throws NoSuchBeanDefinitionException {
        Assert.hasText(beanName, "'beanName' must not be empty");

        BeanDefinition bd = this.beanDefinitionMap.remove(beanName);
        if (bd == null) {
            if (this.logger.isTraceEnabled()) {
                this.logger.trace("No bean named '" + beanName + "' found in " + this);
            }
            throw new NoSuchBeanDefinitionException(beanName);
        }

        if (hasBeanCreationStarted()) {
            // Cannot modify startup-time collection elements anymore (for stable iteration)
            synchronized (this.beanDefinitionMap) {
                List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames);
                updatedDefinitions.remove(beanName);
                this.beanDefinitionNames = updatedDefinitions;
            }
        } else {
            // Still in startup registration phase
            this.beanDefinitionNames.remove(beanName);
        }
        this.frozenBeanDefinitionNames = null;

        resetBeanDefinition(beanName);
    }

    /**
     * Reset all bean definition caches for the given bean,
     * including the caches of beans that are derived from it.
     *
     * @param beanName the name of the bean to reset
     */
    protected void resetBeanDefinition(String beanName) {
        // Remove the merged bean definition for the given bean, if already created.
        clearMergedBeanDefinition(beanName);

        // Remove corresponding bean from singleton cache, if any. Shouldn't usually
        // be necessary, rather just meant for overriding a context's default beans
        // (e.g. the default StaticMessageSource in a StaticApplicationContext).
        destroySingleton(beanName);

        // Reset all bean definitions that have the given bean as parent (recursively).
        for (String bdName : this.beanDefinitionNames) {
            if (!beanName.equals(bdName)) {
                BeanDefinition bd = this.beanDefinitionMap.get(bdName);
                if (beanName.equals(bd.getParentName())) {
                    resetBeanDefinition(bdName);
                }
            }
        }
    }

    /**
     * Only allows alias overriding if bean definition overriding is allowed.
     */
    @Override
    protected boolean allowAliasOverriding() {
        return isAllowBeanDefinitionOverriding();
    }

    @Override
    public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
        super.registerSingleton(beanName, singletonObject);

        if (hasBeanCreationStarted()) {
            // Cannot modify startup-time collection elements anymore (for stable iteration)
            synchronized (this.beanDefinitionMap) {
                if (!this.beanDefinitionMap.containsKey(beanName)) {
                    Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames.size() + 1);
                    updatedSingletons.addAll(this.manualSingletonNames);
                    updatedSingletons.add(beanName);
                    this.manualSingletonNames = updatedSingletons;
                }
            }
        } else {
            // Still in startup registration phase
            if (!this.beanDefinitionMap.containsKey(beanName)) {
                this.manualSingletonNames.add(beanName);
            }
        }

        clearByTypeCache();
    }

    @Override
    public void destroySingleton(String beanName) {
        super.destroySingleton(beanName);
        this.manualSingletonNames.remove(beanName);
        clearByTypeCache();
    }

    @Override
    public void destroySingletons() {
        super.destroySingletons();
        this.manualSingletonNames.clear();
        clearByTypeCache();
    }

    /**
     * Remove any assumptions about by-type mappings.
     */
    private void clearByTypeCache() {
        this.allBeanNamesByType.clear();
        this.singletonBeanNamesByType.clear();
    }


    //---------------------------------------------------------------------
    // Dependency resolution functionality
    //---------------------------------------------------------------------

    @Override
    public <T> NamedBeanHolder<T> resolveNamedBean(Class<T> requiredType) throws BeansException {
        NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, (Object[]) null);
        if (namedBean != null) {
            return namedBean;
        }
        BeanFactory parent = getParentBeanFactory();
        if (parent instanceof AutowireCapableBeanFactory) {
            return ((AutowireCapableBeanFactory) parent).resolveNamedBean(requiredType);
        }
        throw new NoSuchBeanDefinitionException(requiredType);
    }

    @SuppressWarnings("unchecked")
    @Nullable
    private <T> NamedBeanHolder<T> resolveNamedBean(Class<T> requiredType, @Nullable Object... args)
            throws BeansException {
        Assert.notNull(requiredType, "Required type must not be null");
        String[] candidateNames = getBeanNamesForType(requiredType);

        if (candidateNames.length > 1) {
            List<String> autowireCandidates = new ArrayList<>(candidateNames.length);
            for (String beanName : candidateNames) {
                if (!containsBeanDefinition(beanName) || getBeanDefinition(beanName).isAutowireCandidate()) {
                    autowireCandidates.add(beanName);
                }
            }
            if (!autowireCandidates.isEmpty()) {
                candidateNames = StringUtils.toStringArray(autowireCandidates);
            }
        }

        if (candidateNames.length == 1) {
            String beanName = candidateNames[0];
            return new NamedBeanHolder<>(beanName, getBean(beanName, requiredType, args));
        } else if (candidateNames.length > 1) {
            Map<String, Object> candidates = new LinkedHashMap<>(candidateNames.length);
            for (String beanName : candidateNames) {
                if (containsSingleton(beanName) && args == null) {
                    Object beanInstance = getBean(beanName);
                    candidates.put(beanName, (beanInstance instanceof NullBean ? null : beanInstance));
                } else {
                    candidates.put(beanName, getType(beanName));
                }
            }
            String candidateName = determinePrimaryCandidate(candidates, requiredType);
            if (candidateName == null) {
                candidateName = determineHighestPriorityCandidate(candidates, requiredType);
            }
            if (candidateName != null) {
                Object beanInstance = candidates.get(candidateName);
                if (beanInstance == null || beanInstance instanceof Class) {
                    beanInstance = getBean(candidateName, requiredType, args);
                }
                return new NamedBeanHolder<>(candidateName, (T) beanInstance);
            }
            throw new NoUniqueBeanDefinitionException(requiredType, candidates.keySet());
        }

        return null;
    }

    @Override
    @Nullable
    public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
            @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

        descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
        if (Optional.class == descriptor.getDependencyType()) {
            return createOptionalDependency(descriptor, requestingBeanName);
        } else if (ObjectFactory.class == descriptor.getDependencyType() ||
                ObjectProvider.class == descriptor.getDependencyType()) {
            return new DependencyObjectProvider(descriptor, requestingBeanName);
        } else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
            return new Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
        } else {
            Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
                    descriptor, requestingBeanName);
            if (result == null) {
                result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
            }
            return result;
        }
    }

    @Nullable
    public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
            @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

        InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
        try {
            Object shortcut = descriptor.resolveShortcut(this);
            if (shortcut != null) {
                return shortcut;
            }

            Class<?> type = descriptor.getDependencyType();
            Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
            if (value != null) {
                if (value instanceof String) {
                    String strVal = resolveEmbeddedValue((String) value);
                    BeanDefinition bd =
                            (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
                    value = evaluateBeanDefinitionString(strVal, bd);
                }
                TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
                return (descriptor.getField() != null ?
                        converter.convertIfNecessary(value, type, descriptor.getField()) :
                        converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
            }

            Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
            if (multipleBeans != null) {
                return multipleBeans;
            }

            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
            if (matchingBeans.isEmpty()) {
                if (isRequired(descriptor)) {
                    raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
                }
                return null;
            }

            String autowiredBeanName;
            Object instanceCandidate;

            if (matchingBeans.size() > 1) {
                autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
                if (autowiredBeanName == null) {
                    if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
                        return descriptor.resolveNotUnique(type, matchingBeans);
                    } else {
                        // In case of an optional Collection/Map, silently ignore a non-unique case:
                        // possibly it was meant to be an empty collection of multiple regular beans
                        // (before 4.3 in particular when we didn't even look for collection beans).
                        return null;
                    }
                }
                instanceCandidate = matchingBeans.get(autowiredBeanName);
            } else {
                // We have exactly one match.
                Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
                autowiredBeanName = entry.getKey();
                instanceCandidate = entry.getValue();
            }

            if (autowiredBeanNames != null) {
                autowiredBeanNames.add(autowiredBeanName);
            }
            if (instanceCandidate instanceof Class) {
                instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
            }
            Object result = instanceCandidate;
            if (result instanceof NullBean) {
                if (isRequired(descriptor)) {
                    raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
                }
                result = null;
            }
            if (!ClassUtils.isAssignableValue(type, result)) {
                throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
            }
            return result;
        } finally {
            ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
        }
    }

    @Nullable
    private Object resolveMultipleBeans(DependencyDescriptor descriptor, @Nullable String beanName,
            @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) {

        Class<?> type = descriptor.getDependencyType();
        if (type.isArray()) {
            Class<?> componentType = type.getComponentType();
            ResolvableType resolvableType = descriptor.getResolvableType();
            Class<?> resolvedArrayType = resolvableType.resolve();
            if (resolvedArrayType != null && resolvedArrayType != type) {
                type = resolvedArrayType;
                componentType = resolvableType.getComponentType().resolve();
            }
            if (componentType == null) {
                return null;
            }
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType,
                    new MultiElementDescriptor(descriptor));
            if (matchingBeans.isEmpty()) {
                return null;
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.addAll(matchingBeans.keySet());
            }
            TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
            Object result = converter.convertIfNecessary(matchingBeans.values(), type);
            if (getDependencyComparator() != null && result instanceof Object[]) {
                Arrays.sort((Object[]) result, adaptDependencyComparator(matchingBeans));
            }
            return result;
        } else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
            Class<?> elementType = descriptor.getResolvableType().asCollection().resolveGeneric();
            if (elementType == null) {
                return null;
            }
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType,
                    new MultiElementDescriptor(descriptor));
            if (matchingBeans.isEmpty()) {
                return null;
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.addAll(matchingBeans.keySet());
            }
            TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
            Object result = converter.convertIfNecessary(matchingBeans.values(), type);
            if (getDependencyComparator() != null && result instanceof List) {
                ((List<?>) result).sort(adaptDependencyComparator(matchingBeans));
            }
            return result;
        } else if (Map.class == type) {
            ResolvableType mapType = descriptor.getResolvableType().asMap();
            Class<?> keyType = mapType.resolveGeneric(0);
            if (String.class != keyType) {
                return null;
            }
            Class<?> valueType = mapType.resolveGeneric(1);
            if (valueType == null) {
                return null;
            }
            Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType,
                    new MultiElementDescriptor(descriptor));
            if (matchingBeans.isEmpty()) {
                return null;
            }
            if (autowiredBeanNames != null) {
                autowiredBeanNames.addAll(matchingBeans.keySet());
            }
            return matchingBeans;
        } else {
            return null;
        }
    }

    private boolean isRequired(DependencyDescriptor descriptor) {
        return getAutowireCandidateResolver().isRequired(descriptor);
    }

    private boolean indicatesMultipleBeans(Class<?> type) {
        return (type.isArray() || (type.isInterface() &&
                (Collection.class.isAssignableFrom(type) || Map.class.isAssignableFrom(type))));
    }

    @Nullable
    private Comparator<Object> adaptDependencyComparator(Map<String, Object> matchingBeans) {
        Comparator<Object> comparator = getDependencyComparator();
        if (comparator instanceof OrderComparator) {
            return ((OrderComparator) comparator).withSourceProvider(
                    createFactoryAwareOrderSourceProvider(matchingBeans));
        } else {
            return comparator;
        }
    }

    private FactoryAwareOrderSourceProvider createFactoryAwareOrderSourceProvider(Map<String, Object> beans) {
        IdentityHashMap<Object, String> instancesToBeanNames = new IdentityHashMap<>();
        beans.forEach((beanName, instance) -> instancesToBeanNames.put(instance, beanName));
        return new FactoryAwareOrderSourceProvider(instancesToBeanNames);
    }

    /**
     * Find bean instances that match the required type.
     * Called during autowiring for the specified bean.
     *
     * @param beanName the name of the bean that is about to be wired
     * @param requiredType the actual type of bean to look for
     * (may be an array component type or collection element type)
     * @param descriptor the descriptor of the dependency to resolve
     * @return a Map of candidate names and candidate instances that match
     * the required type (never {@code null})
     * @throws BeansException in case of errors
     * @see #autowireByType
     * @see #autowireConstructor
     */
    protected Map<String, Object> findAutowireCandidates(
            @Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {

        String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                this, requiredType, true, descriptor.isEager());
        Map<String, Object> result = new LinkedHashMap<>(candidateNames.length);
        for (Class<?> autowiringType : this.resolvableDependencies.keySet()) {
            if (autowiringType.isAssignableFrom(requiredType)) {
                Object autowiringValue = this.resolvableDependencies.get(autowiringType);
                autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
                if (requiredType.isInstance(autowiringValue)) {
                    result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
                    break;
                }
            }
        }
        for (String candidate : candidateNames) {
            if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
                addCandidateEntry(result, candidate, descriptor, requiredType);
            }
        }
        if (result.isEmpty() && !indicatesMultipleBeans(requiredType)) {
            // Consider fallback matches if the first pass failed to find anything...
            DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
            for (String candidate : candidateNames) {
                if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor)) {
                    addCandidateEntry(result, candidate, descriptor, requiredType);
                }
            }
            if (result.isEmpty()) {
                // Consider self references as a final pass...
                // but in the case of a dependency collection, not the very same bean itself.
                for (String candidate : candidateNames) {
                    if (isSelfReference(beanName, candidate) &&
                            (!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
                            isAutowireCandidate(candidate, fallbackDescriptor)) {
                        addCandidateEntry(result, candidate, descriptor, requiredType);
                    }
                }
            }
        }
        return result;
    }

    /**
     * Add an entry to the candidate map: a bean instance if available or just the resolved
     * type, preventing early bean initialization ahead of primary candidate selection.
     */
    private void addCandidateEntry(Map<String, Object> candidates, String candidateName,
            DependencyDescriptor descriptor, Class<?> requiredType) {

        if (descriptor instanceof MultiElementDescriptor || containsSingleton(candidateName)) {
            Object beanInstance = descriptor.resolveCandidate(candidateName, requiredType, this);
            candidates.put(candidateName, (beanInstance instanceof NullBean ? null : beanInstance));
        } else {
            candidates.put(candidateName, getType(candidateName));
        }
    }

    /**
     * Determine the autowire candidate in the given set of beans.
     * <p>Looks for {@code @Primary} and {@code @Priority} (in that order).
     *
     * @param candidates a Map of candidate names and candidate instances
     * that match the required type, as returned by {@link #findAutowireCandidates}
     * @param descriptor the target dependency to match against
     * @return the name of the autowire candidate, or {@code null} if none found
     */
    @Nullable
    protected String determineAutowireCandidate(Map<String, Object> candidates, DependencyDescriptor descriptor) {
        Class<?> requiredType = descriptor.getDependencyType();
        String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);
        if (primaryCandidate != null) {
            return primaryCandidate;
        }
        String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);
        if (priorityCandidate != null) {
            return priorityCandidate;
        }
        // Fallback
        for (Map.Entry<String, Object> entry : candidates.entrySet()) {
            String candidateName = entry.getKey();
            Object beanInstance = entry.getValue();
            if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||
                    matchesBeanName(candidateName, descriptor.getDependencyName())) {
                return candidateName;
            }
        }
        return null;
    }

    /**
     * Determine the primary candidate in the given set of beans.
     *
     * @param candidates a Map of candidate names and candidate instances
     * (or candidate classes if not created yet) that match the required type
     * @param requiredType the target dependency type to match against
     * @return the name of the primary candidate, or {@code null} if none found
     * @see #isPrimary(String, Object)
     */
    @Nullable
    protected String determinePrimaryCandidate(Map<String, Object> candidates, Class<?> requiredType) {
        String primaryBeanName = null;
        for (Map.Entry<String, Object> entry : candidates.entrySet()) {
            String candidateBeanName = entry.getKey();
            Object beanInstance = entry.getValue();
            if (isPrimary(candidateBeanName, beanInstance)) {
                if (primaryBeanName != null) {
                    boolean candidateLocal = containsBeanDefinition(candidateBeanName);
                    boolean primaryLocal = containsBeanDefinition(primaryBeanName);
                    if (candidateLocal && primaryLocal) {
                        throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
                                "more than one 'primary' bean found among candidates: " + candidates.keySet());
                    } else if (candidateLocal) {
                        primaryBeanName = candidateBeanName;
                    }
                } else {
                    primaryBeanName = candidateBeanName;
                }
            }
        }
        return primaryBeanName;
    }

    /**
     * Determine the candidate with the highest priority in the given set of beans.
     * <p>Based on {@code @javax.annotation.Priority}. As defined by the related
     * {@link org.springframework.core.Ordered} interface, the lowest value has
     * the highest priority.
     *
     * @param candidates a Map of candidate names and candidate instances
     * (or candidate classes if not created yet) that match the required type
     * @param requiredType the target dependency type to match against
     * @return the name of the candidate with the highest priority,
     * or {@code null} if none found
     * @see #getPriority(Object)
     */
    @Nullable
    protected String determineHighestPriorityCandidate(Map<String, Object> candidates, Class<?> requiredType) {
        String highestPriorityBeanName = null;
        Integer highestPriority = null;
        for (Map.Entry<String, Object> entry : candidates.entrySet()) {
            String candidateBeanName = entry.getKey();
            Object beanInstance = entry.getValue();
            if (beanInstance != null) {
                Integer candidatePriority = getPriority(beanInstance);
                if (candidatePriority != null) {
                    if (highestPriorityBeanName != null) {
                        if (candidatePriority.equals(highestPriority)) {
                            throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
                                    "Multiple beans found with the same priority ('" + highestPriority +
                                            "') among candidates: " + candidates.keySet());
                        } else if (candidatePriority < highestPriority) {
                            highestPriorityBeanName = candidateBeanName;
                            highestPriority = candidatePriority;
                        }
                    } else {
                        highestPriorityBeanName = candidateBeanName;
                        highestPriority = candidatePriority;
                    }
                }
            }
        }
        return highestPriorityBeanName;
    }

    /**
     * Return whether the bean definition for the given bean name has been
     * marked as a primary bean.
     *
     * @param beanName the name of the bean
     * @param beanInstance the corresponding bean instance (can be null)
     * @return whether the given bean qualifies as primary
     */
    protected boolean isPrimary(String beanName, Object beanInstance) {
        if (containsBeanDefinition(beanName)) {
            return getMergedLocalBeanDefinition(beanName).isPrimary();
        }
        BeanFactory parent = getParentBeanFactory();
        return (parent instanceof DefaultListableBeanFactory &&
                ((DefaultListableBeanFactory) parent).isPrimary(beanName, beanInstance));
    }

    /**
     * Return the priority assigned for the given bean instance by
     * the {@code javax.annotation.Priority} annotation.
     * <p>The default implementation delegates to the specified
     * {@link #setDependencyComparator dependency comparator}, checking its
     * {@link OrderComparator#getPriority method} if it is an extension of
     * Spring's common {@link OrderComparator} - typically, an
     * {@link org.springframework.core.annotation.AnnotationAwareOrderComparator}.
     * If no such comparator is present, this implementation returns {@code null}.
     *
     * @param beanInstance the bean instance to check (can be {@code null})
     * @return the priority assigned to that bean or {@code null} if none is set
     */
    @Nullable
    protected Integer getPriority(Object beanInstance) {
        Comparator<Object> comparator = getDependencyComparator();
        if (comparator instanceof OrderComparator) {
            return ((OrderComparator) comparator).getPriority(beanInstance);
        }
        return null;
    }

    /**
     * Determine whether the given candidate name matches the bean name or the aliases
     * stored in this bean definition.
     */
    protected boolean matchesBeanName(String beanName, @Nullable String candidateName) {
        return (candidateName != null &&
                (candidateName.equals(beanName) || ObjectUtils.containsElement(getAliases(beanName), candidateName)));
    }

    /**
     * Determine whether the given beanName/candidateName pair indicates a self reference,
     * i.e. whether the candidate points back to the original bean or to a factory method
     * on the original bean.
     */
    private boolean isSelfReference(@Nullable String beanName, @Nullable String candidateName) {
        return (beanName != null && candidateName != null &&
                (beanName.equals(candidateName) || (containsBeanDefinition(candidateName) &&
                        beanName.equals(getMergedLocalBeanDefinition(candidateName).getFactoryBeanName()))));
    }

    /**
     * Raise a NoSuchBeanDefinitionException or BeanNotOfRequiredTypeException
     * for an unresolvable dependency.
     */
    private void raiseNoMatchingBeanFound(
            Class<?> type, ResolvableType resolvableType, DependencyDescriptor descriptor) throws BeansException {

        checkBeanNotOfRequiredType(type, descriptor);

        throw new NoSuchBeanDefinitionException(resolvableType,
                "expected at least 1 bean which qualifies as autowire candidate. " +
                        "Dependency annotations: " + ObjectUtils.nullSafeToString(descriptor.getAnnotations()));
    }

    /**
     * Raise a BeanNotOfRequiredTypeException for an unresolvable dependency, if applicable,
     * i.e. if the target type of the bean would match but an exposed proxy doesn't.
     */
    private void checkBeanNotOfRequiredType(Class<?> type, DependencyDescriptor descriptor) {
        for (String beanName : this.beanDefinitionNames) {
            RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            Class<?> targetType = mbd.getTargetType();
            if (targetType != null && type.isAssignableFrom(targetType) &&
                    isAutowireCandidate(beanName, mbd, descriptor, getAutowireCandidateResolver())) {
                // Probably a proxy interfering with target type match -> throw meaningful exception.
                Object beanInstance = getSingleton(beanName, false);
                Class<?> beanType = (beanInstance != null && beanInstance.getClass() != NullBean.class) ?
                                    beanInstance.getClass() : predictBeanType(beanName, mbd);
                if (beanType != null && !type.isAssignableFrom(beanType)) {
                    throw new BeanNotOfRequiredTypeException(beanName, type, beanType);
                }
            }
        }

        BeanFactory parent = getParentBeanFactory();
        if (parent instanceof DefaultListableBeanFactory) {
            ((DefaultListableBeanFactory) parent).checkBeanNotOfRequiredType(type, descriptor);
        }
    }

    /**
     * Create an {@link Optional} wrapper for the specified dependency.
     */
    private Optional<?> createOptionalDependency(
            DependencyDescriptor descriptor, @Nullable String beanName, final Object... args) {

        DependencyDescriptor descriptorToUse = new NestedDependencyDescriptor(descriptor) {
            @Override
            public boolean isRequired() {
                return false;
            }

            @Override
            public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory) {
                return (!ObjectUtils.isEmpty(args) ? beanFactory.getBean(beanName, args) :
                        super.resolveCandidate(beanName, requiredType, beanFactory));
            }
        };
        return Optional.ofNullable(doResolveDependency(descriptorToUse, beanName, null, null));
    }


    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder(ObjectUtils.identityToString(this));
        sb.append(": defining beans [");
        sb.append(StringUtils.collectionToCommaDelimitedString(this.beanDefinitionNames));
        sb.append("]; ");
        BeanFactory parent = getParentBeanFactory();
        if (parent == null) {
            sb.append("root of factory hierarchy");
        } else {
            sb.append("parent: ").append(ObjectUtils.identityToString(parent));
        }
        return sb.toString();
    }


    //---------------------------------------------------------------------
    // Serialization support
    //---------------------------------------------------------------------

    private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
        throw new NotSerializableException("DefaultListableBeanFactory itself is not deserializable - " +
                "just a SerializedBeanFactoryReference is");
    }

    protected Object writeReplace() throws ObjectStreamException {
        if (this.serializationId != null) {
            return new SerializedBeanFactoryReference(this.serializationId);
        } else {
            throw new NotSerializableException("DefaultListableBeanFactory has no serialization id");
        }
    }


    /**
     * Minimal id reference to the factory.
     * Resolved to the actual factory instance on deserialization.
     */
    private static class SerializedBeanFactoryReference implements Serializable {

        private final String id;

        public SerializedBeanFactoryReference(String id) {
            this.id = id;
        }

        private Object readResolve() {
            Reference<?> ref = serializableFactories.get(this.id);
            if (ref != null) {
                Object result = ref.get();
                if (result != null) {
                    return result;
                }
            }
            // Lenient fallback: dummy factory in case of original factory not found...
            return new DefaultListableBeanFactory();
        }
    }

    private static class NestedDependencyDescriptor extends DependencyDescriptor {

        public NestedDependencyDescriptor(DependencyDescriptor original) {
            super(original);
            increaseNestingLevel();
        }
    }

    private static class MultiElementDescriptor extends NestedDependencyDescriptor {

        public MultiElementDescriptor(DependencyDescriptor original) {
            super(original);
        }
    }

    /**
     * Serializable ObjectFactory/ObjectProvider for lazy resolution of a dependency.
     */
    private class DependencyObjectProvider implements ObjectProvider<Object>, Serializable {

        private final DependencyDescriptor descriptor;

        private final boolean optional;

        @Nullable
        private final String beanName;

        public DependencyObjectProvider(DependencyDescriptor descriptor, @Nullable String beanName) {
            this.descriptor = new NestedDependencyDescriptor(descriptor);
            this.optional = (this.descriptor.getDependencyType() == Optional.class);
            this.beanName = beanName;
        }

        @Override
        public Object getObject() throws BeansException {
            if (this.optional) {
                return createOptionalDependency(this.descriptor, this.beanName);
            } else {
                Object result = doResolveDependency(this.descriptor, this.beanName, null, null);
                if (result == null) {
                    throw new NoSuchBeanDefinitionException(this.descriptor.getResolvableType());
                }
                return result;
            }
        }

        @Override
        public Object getObject(final Object... args) throws BeansException {
            if (this.optional) {
                return createOptionalDependency(this.descriptor, this.beanName, args);
            } else {
                DependencyDescriptor descriptorToUse = new DependencyDescriptor(descriptor) {
                    @Override
                    public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory) {
                        return beanFactory.getBean(beanName, args);
                    }
                };
                Object result = doResolveDependency(descriptorToUse, this.beanName, null, null);
                if (result == null) {
                    throw new NoSuchBeanDefinitionException(this.descriptor.getResolvableType());
                }
                return result;
            }
        }

        @Override
        @Nullable
        public Object getIfAvailable() throws BeansException {
            if (this.optional) {
                return createOptionalDependency(this.descriptor, this.beanName);
            } else {
                DependencyDescriptor descriptorToUse = new DependencyDescriptor(descriptor) {
                    @Override
                    public boolean isRequired() {
                        return false;
                    }
                };
                return doResolveDependency(descriptorToUse, this.beanName, null, null);
            }
        }

        @Override
        @Nullable
        public Object getIfUnique() throws BeansException {
            DependencyDescriptor descriptorToUse = new DependencyDescriptor(descriptor) {
                @Override
                public boolean isRequired() {
                    return false;
                }

                @Override
                @Nullable
                public Object resolveNotUnique(Class<?> type, Map<String, Object> matchingBeans) {
                    return null;
                }
            };
            if (this.optional) {
                return createOptionalDependency(descriptorToUse, this.beanName);
            } else {
                return doResolveDependency(descriptorToUse, this.beanName, null, null);
            }
        }

        @Nullable
        protected Object getValue() throws BeansException {
            if (this.optional) {
                return createOptionalDependency(this.descriptor, this.beanName);
            } else {
                return doResolveDependency(this.descriptor, this.beanName, null, null);
            }
        }
    }

    /**
     * Serializable ObjectFactory for lazy resolution of a dependency.
     */
    private class Jsr330DependencyProvider extends DependencyObjectProvider implements Provider<Object> {

        public Jsr330DependencyProvider(DependencyDescriptor descriptor, @Nullable String beanName) {
            super(descriptor, beanName);
        }

        @Override
        @Nullable
        public Object get() throws BeansException {
            return getValue();
        }
    }

    /**
     * Separate inner class for avoiding a hard dependency on the {@code javax.inject} API.
     */
    private class Jsr330ProviderFactory {

        public Object createDependencyProvider(DependencyDescriptor descriptor, @Nullable String beanName) {
            return new Jsr330DependencyProvider(descriptor, beanName);
        }
    }

    /**
     * An {@link org.springframework.core.OrderComparator.OrderSourceProvider} implementation
     * that is aware of the bean metadata of the instances to sort.
     * <p>Lookup for the method factory of an instance to sort, if any, and let the
     * comparator retrieve the {@link org.springframework.core.annotation.Order}
     * value defined on it. This essentially allows for the following construct:
     */
    private class FactoryAwareOrderSourceProvider implements OrderComparator.OrderSourceProvider {

        private final Map<Object, String> instancesToBeanNames;

        public FactoryAwareOrderSourceProvider(Map<Object, String> instancesToBeanNames) {
            this.instancesToBeanNames = instancesToBeanNames;
        }

        @Override
        @Nullable
        public Object getOrderSource(Object obj) {
            RootBeanDefinition beanDefinition = getRootBeanDefinition(this.instancesToBeanNames.get(obj));
            if (beanDefinition == null) {
                return null;
            }
            List<Object> sources = new ArrayList<>(2);
            Method factoryMethod = beanDefinition.getResolvedFactoryMethod();
            if (factoryMethod != null) {
                sources.add(factoryMethod);
            }
            Class<?> targetType = beanDefinition.getTargetType();
            if (targetType != null && targetType != obj.getClass()) {
                sources.add(targetType);
            }
            return sources.toArray(new Object[sources.size()]);
        }

        @Nullable
        private RootBeanDefinition getRootBeanDefinition(@Nullable String beanName) {
            if (beanName != null && containsBeanDefinition(beanName)) {
                BeanDefinition bd = getMergedBeanDefinition(beanName);
                if (bd instanceof RootBeanDefinition) {
                    return (RootBeanDefinition) bd;
                }
            }
            return null;
        }
    }

}
